The present invention relates to a method of improving the contrast of images obtained using the pulsed image-addition ESPI technique.
When laser sources first came into use in the 60s, a curious phenomenon known as the speckle effect was observed, and which is produced when the surface of an object of a roughness comparable with the wavelength of visible light (450-700 nm) is illuminated by a beam of coherent light (e.g. a laser beam). In which case, the surface of the object assumes a typical granular appearance of randomly distributed light and dark specks. The speckle effect is caused by multiple interference of the object-diffused fields, which have randomly distributed phases on account of the roughness of the object being comparable with the wavelength, and is extremely difficult to analyze theoretically, mainly on account of the statistical characteristics of the roughness of the object and the coherence properties of the light used. Moreover, the statistical distribution of the luminous intensity of a speckle image has no direct relationship with the microscopic structure of the rough surface gene rating the image.
The ESPI (Electronic Speckle Pattern Interferometry) technique, which is known in its more general form, uses the speckle effect to accurately real-time monitor the deformation of mechanically stressed objects. More specifically, by subtracting or adding successive speckle images, the ESPI technique generates interference images showing correlation fringes, the arrangement of which is related instant by instant to the deformation of the object.
In the speckle image subtraction process, interference images are generated by first illuminating a substantially flat surface of the object by means of a laser beam to detect and digitize a first speckle image of the surface of the undeformed object; the object is then stressed mechanically and a second speckle image detected of the deformed object; and the second image is compared electronically with the first (e.g. by subtracting the grey levels of corresponding points of the two images) to obtain a final image (interferogram) showing correlation fringes of increasing density in the regions of the object undergoing maximum deformation. The luminous intensity of each point of the interferogram is given by the following equation:                               I          ⁡                      (                          x              ,              y                        )                          =                  4          ⁢                                                    I                0                            ⁢                              I                R                                              ⁢                      "LeftBracketingBar"                          sin              ⁡                              (                                  Φ                  +                                      ΔΦ                    2                                                  )                                      "RightBracketingBar"                    ⁢                      "LeftBracketingBar"                          sin              ⁡                              (                                  ΔΦ                  2                                )                                      "RightBracketingBar"                                              (        1        )            
where I0 is the intensity of the light backscattered by the object; IR is the intensity of a reference beam detected simultaneously with the backscattered light; "PHgr"(x,y) is the random phase relative to distribution of the speckle light; and "PHgr"xcex94(x,y) is the phase variation relative to the variation in the optical path generated by surface deformation of the object.
The above equation provides for determining actual deformation of the object from the correlation fringe pattern.
In the speckle image addition process, interference images are generated by adding, as opposed to subtracting, the two speckle images to obtain an interferogram similar to that of the subtraction process, but which is characterized by poor contrast of the fringes, and which is governed by the following equation:                               I          ⁡                      (                          x              ,              y                        )                          =                              2            ⁢                          (                                                I                  0                                +                                  I                  R                                            )                                +                      4            ⁢                                                            I                  0                                ⁢                                  I                  R                                                      ⁢                          "LeftBracketingBar"                                                cos                  ⁡                                      (                                          Φ                      +                                              ΔΦ                        2                                                              )                                                  ⁢                                  cos                  ⁡                                      (                                          ΔΦ                      2                                        )                                                              "RightBracketingBar"                                                          (        2        )            
As can be seen, in addition to a phase shift of the fringes as compared with those obtained using the subtraction process (so that maximum luminosity of the image-addition interferogram corresponds to minimum luminosity of that of the image-subtraction process), the image-addition interferogram also differs by comprising noise term 2(I0+IR), which represents a disturb term greatly reducing visibility (and therefore contrast) of the fringes.
By way of a solution to the problem, an alternative technique has been proposed whereby two successive interference images obtained using the addition process are subtracted one from the other to obtain further images of a luminous intensity according to the following equation:                               I          ⁡                      (                          x              ,              y                        )                          =                  "LeftBracketingBar"                                    2              ⁢                                                                    I                    0                                    ⁢                                      I                    R                                                              ⁢              cos              ⁢                              xe2x80x83                            ⁢              Φ                        -                          2              ⁢                                                                    I                    0                                    ⁢                                      I                    R                                                              ⁢                              cos                ⁡                                  (                                      Φ                    +                    α                    +                                          ΔΦ                      2                                                        )                                            ⁢              cos              ⁢                              ΔΦ                2                                              "RightBracketingBar"                                    (        3        )            
where xcex1 is the phase variation generated between the instants in which the two interference images are formed.
While improving contrast of the fringes, the above technique nevertheless still involves a random noise term 2{square root over (I0+L IR+L )} cos "PHgr", and a second set of fringes is formed due to the presence of term xcex1.
ESPI measurements to study the deformation of mechanically stressed objects may be made using interferometers of different optical configurations for measuring in-plane or out-of-plane deformation, as required, i.e. for determining deformation of the object in or outside the plane of the monitored surface (assuming the surface is substantially flat).
The above considerations also apply to ESPI measurements made using a continuously operating or pulsed laser. That is, the stressed object is subjected to laser pulses at a predetermined frequency to generate respective speckle images, which are detected and displayed, and which may also be subjected to addition and subtraction processes to obtain interferograms. Pulsed laser measurements provide for studying particularly rapid deformation processes by enabling comparison of closely succeeding deformation states (corresponding to the instants in which the laser pulses are emitted).
It is an object of the present invention to provide a method of improving the fringe visibility of ESPI measurements made using a pulsed source and the image-addition process.
According to the present invention, there is provide a method of improving the contrast of images obtained using the pulsed image-addition ESPI technique, and as described in claim 1.